A conventional object detecting apparatus disposed on a vehicle is disclosed in a Japanese Patent Document JP-A-2002-031685. The apparatus comprises a light emitting device that emits a laser beam, a polygonal mirror in a shape of a six-sided horizontally-sliced pyramid that rotatably reflects the laser beam, and a light receiving device that picks up reflection of the laser beam by an object. The apparatus projects the laser beam by reflecting by the polygonal mirror toward a space in a front area of the vehicle. The laser beam reflected by the polygonal mirror is so controlled that it scans a predetermined area of space in the front area of the vehicle. Reflection of the laser beam from, for example, a preceding vehicle is received by the light receiving device and gives a measurement of distance to the vehicle based on an elapsed time between projection and reception of the beam.
The object detecting apparatus may give a false measurement of distance when a sensor surface such as a light projection window for laser beam projection or the like has foreign matter such as dirt that interferes with the laser beam. In addition, when the object detecting apparatus is disposed behind a front cover (or a front grille) of a vehicle, dirt on the cover has the same effect as the dirt on the light projection window.
Therefore, the foreign matter in a path of the laser beam, as shown in a schematic diagram of FIG. 6, is detected by collecting reflection of the laser beam reflected by the foreign matter. That is, the laser beam projected toward a front space of a vehicle from a light-emitting diode J2 in a case J1 is reflected by the foreign matter on a sensor surface J3 and by the foreign matter on a surface of a front cover J4 of a vehicle, and is detected by light sensing elements J5 and J6, respectively.
However, in the conventional apparatus, two light sensing elements J5 and J6 are employed to detect the dirt on the sensor surface J3 and the dirt on the front cover J4. The redundancy of the sensing elements leads to a problem that an increased number of parts for the apparatus and an increased cost of production are required.
The object detecting apparatus is typically installed in a hole or the like disposed on a front bumper of a vehicle. Installation of the apparatus in the hole of the front bumper worsens a styling of the vehicle. In order not to worsen the styling, various positions of installation are proposed. In one installation, the apparatus is disposed behind a front grille, and the laser beam passes between two crosspieces of the front grille.
The position of the apparatus behind the grille equates the grille with the front cover in the schematic diagram of FIG. 6. That is, dirt on the grille may interfere with the laser beam and may result in a false measurement of distance.
Therefore, it is necessary to detect the dirt on the grille in order to have a correct measurement of distance. This situation leads to the same problem that two light sensing elements being disposed besides the light receiving device for distance measurement have to be employed to detect the reflection of the laser beam from both of the dirt on the sensor surface and the dirt on the grille surface.